Any mini-vial containing radioactive substances in liquid form therein requires special structural supports when used on conveyors in liquid scintillation counters, and as such detracts from the performance of the mini-vial by increasing the unwanted background and decreasing the optical clarity and therefore the counting efficiency.
Liquid scintillation counting of radioactive materials is accomplished via interaction of a radioactive sample with a cocktail system consisting of fluorescent compounds suspended in solvents within a liquid scintillation vial. This interaction produces visible light photons (proportional to the radiation intensity) which are counted by the LSC apparatus.
Liquid scintillation vials are usually composed of glass or polyethylene, but are also available in quartz, nylon, and teflon models. Most scintillation counters are automated sample changer systems, with changers standardized to accept vials which contain a maximum of approximately 20 ml. of sample and cocktail and have dimensions of 25-30 mm. diameter by 55-65 mm. height.
Since the fluors, vials, and solvents are relatively expensive and usually disposed of after a single use, miniature vials (mini-vials) have been developed. The vials hold approximately 2-7 ml. of cocktail and are approximately 10-18 mm. in diameter and 50-65 mm. in height. These vials are available in the same materials as the aforementioned, and also are available in the form of plastic bags which are heat-sealed after addition of the sample and cocktail.
Since the liquid scintillation process relies on the transmission of light through a vial, the composition and structure of the vials effect the performance of the system. The optical clarity of the vial effects the relative counting efficiency, so clear glass vials usually have better counting efficiency than polyethylene vials. Naturally occurring radioisotopes such as Potassium 40 contribute unwanted radioactive counts (background) to the vials, however, and are most abundant in glass vials. Polyethylene vials have lower backgrounds than glass vials, but are more opaque and have lower efficiencies.
Mini-vials cannot be used in most liquid scintillation counters without adapters which are of the same physical dimensions as the large vials the counters are designed to use. This mini-vial/adapter system, by virtue of the adapter material (glass, plastic, etc.) results in a decrease in the optical clarity of the counting system (decreased efficiency) and a concomitant increase in background. (Heat-sealed bag systems always requires these adapters, and mini-vials require adapters unless being used in counters designed for mini-vials only.) These adapters also add increased work to the sample preparation/counting/clean-up process. Because large vials and mini-vials have different physical dimensions, diameter related effects introduce errors such that results obtained with the two types of vials cannot accurately be directly intercompared.
A final disadvantage to present-day vials is that static electricity affects the counting vials. Peizo Electric Generators, radioactive external alpha sources, and staticidal sprays are employed to eliminate static charges on vials. When mini-vials are used, the requisite adapters diminish the usefulness of these static eliminators.
It was to overcome these inherent defects that the present invention was conceived for liquid scintillation.